1. Technical Field
This invention relates to high-pressure discharge lamps having a discharge vessel enclosed by an outer bulb, and more particularly to a metal halide lamp having a starting aid arranged in the intervening space between the outer bulb and the discharge vessel.
2. Description of the Related Art
High pressure discharge lamps, or more particularly metal halide lamps, having starting aids are known in the art. Such lamps are suitable for various applications such as general interior lighting, general exterior lighting, video illumination, etc. The discharge vessel of the known lamp is typically made of quartz glass. Alternatively, this vessel may be made of a ceramic material, herein understood to be a densely sintered polycrystalline metal oxide such as Al.sub.2 O.sub.3 or YAG or a densely sintered polycrystalline metal nitride such as AlN.
A known problem of metal halide lamps is the comparatively wide spread in ignition time, which arises from a shortage of free electrons due to the presence of electronegative iodine in the lamp filling. Several methods are known in the art to counteract this problem. For example, the addition of a small quantity of .sup.85 Kr in the discharge vessel can supplement such a shortage. A disadvantage of .sup.85 Kr as a filling material is its radioactivity.
Alternatively, ignition aids, such as a UV-enhancer, are used in metal halide lamps to promote ignition. A UV-enhancer is typically a small discharge tube positioned adjacent the discharge vessel that acts as an ultraviolet radiation source. Such a UV-enhancer has been disclosed in U.S. Pat. No. 4,818,915 to Zaslavsky et al. This UV-enhancer has an envelope of UV-transmitting quartz material. Upon breakdown, the UV-enhancer will generate UV-radiation at about 253.7 nm or less. The influence of this UV-radiation leads to the production of free electrons in the discharge vessel, which in turn strongly promotes lamp ignition.
Upon application of an ignition pulse supplied by a ballast, UV enhancers capacitively couple energy from one lamp terminal to the other through the UV enhancer gas (or Penning mixture). The UV enhancer must be positioned to provide a minimum gap between the quartz body of the UV enhancer and the opposite potential. This gap reduces arcing through the UV enhancer body which may destroy the UV enhancer itself and the lamp. Furthermore, the orientation between the electrode and emitting plate within the UV enhancer and the opposite potential has an effect on the minimum voltage necessary to create a glow voltage. Thus, for optimum performance, assembly of the lamp requires care in alignment of the UV enhancer's emitting surface with respect to the opposite potential.
Current UV enhancer designs provide no frame support because the enhancers are supported at one end only. Thus, in some lamp types, a separate bridge support structure is necessary.